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MEG Response to Band-Passed Noise

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MEG Response toBand-Passed NoiseJulien DagenaisWhat is MEG? Magnetoencephelography (MEG) is the measurement ofextracranial magnetic fields produced by electrical currentswithin the brain In MEG recordings, weak magnetic fields outside the head aredetected with an array of sensors, and on the basis of themeasured signals, the underlying cerebral currents areestimated. Since cerebral magnetic fields are extremely weak whencompared, for example, with the Earth's magnetic field, specialdevices are needed to measure them. Development of sensitiveSQUID (Superconducting Quantum Interference Device) sensorsallows the detection of small changes in the magnetic fields.Purpose of Experiment Compared to the visual system, the functionalorganization of the human auditory cortex isnot well understood Research on non-human primate anatomyand physiology as well as recent humanneuro-imaging work has generally skippedover studies on band-passed noise This experiment attempts to understand howthe brain reacts to band-passed noise atdifferent levels of bandwidthDefining a Peak WidthLatency/AmplitudeDefining the M Peaks M50: defined sink/source orientation withpeak at ~35-70 ms M50b: same orientation but appearanceanterior to the M50 and at a ~15ms delay M100: opposite orientation with peak at ~90-160 ms M150: same orientation as M50 withappearance at ~145-195 msThe M50b Found in pure tone in just 1 of 24 cases Found almost exclusively in band-passed noise (18 of 24 cases) 65ms 91msMethodology 5 stimuli: 1000 hz tone, ¼ oct, ½ oct, 1 oct,2 octs with center frequencies of 1000 hz 100 ms duration, 5 ms ramps 1000 repetitions Processing: 1) De-Noising 2) Epoch and Concatenation 3) LPF at 30 hz 4) Pre-trigger baseline correctionDefining the Channels R0020 Localizer RH: 5 best Sink/SourceResults As bandwidth increases in the M100, amplitudedecreases (with exception at 2 octaves) The M50 is amplitude-dominant in the LH M50b is consistently present solely in band-passednoise, effectively becoming more pronounced asbandwidth increases Waveform peak responses to noise are contextdependant – the presence of noise causes asignificant delay to the M50/M100/M150 for puretone The location of these dipoles are decidedly anterior tothe localizer dipolesA Typical ResponseExamining the Peaks0204060801001201401601802000 1 2 3 4 5 M50 M50b M100 M150WidthLatencyAmplitudeLatency vs. ConditionFew clear trends in bandwidthLatency vs Condition (RH M100)Blue: R0020 - Red: R0037 - Yellow: R00450204060801001201401601800 1 2 3 4 5 6Condition LatencyPure Tone1/4 Octave1/2 Octave1 Octave2 OctaveLocalizerAverageAmplitude Vs ConditionAmplitude vs. Condition (RH M100)Blue: R0020 - Red: R0037 - Yellow: R00450204060801001201400 1 2 3 4 5 6Condition AmplitudePure Tone1/4 Octave1/2 Octave1 Octave2 OctaveLocalizerAverageVs the Localizer (SpatialDomain)Localizer Dipoles in Red, Pure Tone in OrangeThe Delayed ResponseM50M100 R0045 LH ½ OctaveM50M100Vs the Localizer (Time Domain)R0020 LH Localizer R0020 LH 2 OctaveM50 (56 ms)M100 (98 ms)M50 (79 ms)M50b (100ms)M100 (166 ms)R0020 LH Pure ToneM50 (68ms)M50b (95 ms)M100 (139 ms)The M50b Width and Amplitude increase with bandwidth ¼ Octave 1 Octave 2 OctaveM50bM50bM50bM50b Width/AmplitudeThe M50b051015202530350 1 2 3 41/4 Octave 1 Octave 2 OctaveWidthAmplitudeAmplitude (LH - RH M50 across all conditions/subjects)-35-25-15-551525350 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16AmplitudeLH-RHM50 LH-RH Amplitude2 OctavesM50 LH vs RH AmplitudeM100 LH-RH LatencyLatency (LH-RH M100 across all conditions/subjects)-35-25-15-551525350 2 4 6 8 10 12LatencyLH-RHM100 LH vs RH LatencyLHRHTrendsLatency vs Amplitude (LH All Subjects)0501001502002500 10 20 30 40 50 60 70 80AmplitudeLatencyM50 R0020M50b R0020M100 R0020M150 R0020M50 R0037M50b R0037M100 R0037M150 R0037M50 R0045M100 R0045M100 largest spread in amplitude (and width)M150 largest spread in latencyTrendsAmplitude vs Width (RH all)0102030405060700 10 20 30 40 50 60 70WidthAmplitudeM50M50bM100M150Conclusions Dominance of LH M50 consistent with previousstudies on all types of band-passed noise M50b is consistently present solely in band-passednoise, effectively becoming more pronounced asbandwidth increases Waveform peak responses to noise are contextdependant – the presence of noise causes asignificant delay to the M50/M100/M150 for puretone The location of these dipoles are decidedly anterior tothe localizer dipolesThanksJonathan Z. SimonDavid PoeppelMaria ChaitNayef AhmarShantanu RayJuanJuan


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